This invention relates to vacuum pumping systems and more particularly to a combination of mechanical booster pumps and mechanical backing pumps, suitable for evacuating an enclosure.
A typical oil lubricant free of dry pump is disclosed and described in our UK Patent Specification No. 2,088,957. Such a pump comprises three pumping chambers, two chambers or which, at the outlet end of the pump, can contain intermeshing pairs of rotors of the `Claw` type with a third chamber at the inlet end of the pump containing intermeshing rotors of the `Roots` type. A pair of shafts effective to drive the rotor pairs are supported for rotation at either side of the pump by end bearings enclosed in respective sealed housings and are interconnected by meshing gears encased for required synchronisation of rotation in a housing at one end of the pump casing; one of the shafts extends beyond the housing for connection to a prime mover such as an electric motor.
Oil free mechanical pumps of this type provide a high volumetric pumping efficiency and are generally capable of evacuating an enclosure to a pressure of the order of 10.sup.-2 torr.
The absence of lubricant within the pumping chambers of such pumps makes them particularly suitable for applications in which an extremely low level of contamination, particularly from gear train lubricants and end bearing lubricants, is essential and has to be combined with a low ultimate pressure.
To achieve high pumping capacity and pressures below 10.sup.-2 torr, for example pressures of the order of 10.sup.-3 torr, it has been proposed to use such an oil free mechanical vacuum pump in combination with a booster pump of known kind. Such a booster pump which generally also is an oil free pump, interfaces with the enclosure to be evacuated and is backed by an oil free pump of the type described in the above UK patent.
Booster pumps of known kind generally can have a construction similar to that of the oil free mechanical pumps described above. Such pumps typically also comprise at least one chamber with intermeshing rotor pairs mounted upon shafts and supported for rotation at opposite ends of the pump by end bearings enclosed in respective sealed housings. One such housing contains intermeshing gears for producing the required synchronized rotation of the shafts while both housings contain lubricant for the gears and for the shaft bearings.
To ensure rapid evacuation of the pump and enclosure, internal gas passages are provided between each housing and the pumping chamber or chambers of the pump. These passages are convoluted and shielded to ensure that lubricants and impurities from the housings are inhibited from entering the pumping chamber.
One serious disadvantage of such booster pumps arises from the inability completely to stop penetration of lubricant, and in particular lubricant vapour, through these passages and in to the pumping chambers of the pump. This applies in particular to lubricants such as perfluoropolyether (PFPE) lubricants. Such lubricants could ultimately reach the enclosure being pumped by a combination of back diffusion and transfer from rotor to rotor and thereby considerably increase the level of contamination in the enclosure. In many cases, particularly for applications requiring a clean environment, the amount of contamination so carried over into the pumped enclosure can reach unacceptable levels.
The present invention is concerned with a reduction in the amount of contamination in the enclosure being pumped arising in particular from lubricants and other impurities present in the bearing housings of such pumps.